1. Field of the Invention
The present invention relates to high bypass turbofan engines and, more specifically, to a high bypass turbofan engine having a fan drive turbine which is partially geared to match the speed of rotation of the fan.
2. Description of the Related Art As the bypass ratio of a turbofan engine increases, the amount of air moved by the fan section increases relative to the amount of air received by the core engine. In such a situation the fan section diameter becomes larger relative to the core diameter. As the fan diameter increases the optimum tip speed of the fan decreases. The combination of the larger diameter fan section and the requirement for a lower fan tip speed tends to cause the required number of fan drive turbine stages to increase due to the lower turbine rotational speed and the thermodynamic power cycle of the turbine, thus increasing the cost and weight of the engine. The high power extraction from the turbine stream requires large expansion of the turbine gas and hence a large turbine exit annulas area.
The conventional solution to this problem was to insert a speed changing gear box between the fan drive turbine section and the fan section. However, such an arrangement has substantial drawbacks. The fan drive turbine though requiring fewer stages still requires the same large exit annulus area due to the power extraction requirement for the thermodynamic cycle of the turbine, and, therefore, its speed is limited by stress in the last stage of the fan drive turbine. Hence, the turbine remains at a relatively large diameter with a significant number of stages. Secondly, the gear box between the fan drive turbine and the fan section is relatively heavy, inefficient, expensive and requires substantial maintenance due to its large shaft horsepower output and lower output speed.